An R—Fe—B based rare-earth magnet (where R is a rare-earth element, Fe is iron, and B is boron) is a typical high-performance permanent magnet, has a structure including, as a main phase, an R2Fe14B phase, which is a ternary tetragonal compound, and exhibits excellent magnet performance. Such R—Fe—B based rare-earth magnets are roughly classifiable into sintered magnets and bonded magnets. A sintered magnet is produced by compacting a fine powder of an R—Fe—B based magnet alloy (with a mean particle size of several μm) with a press machine and then sintering the resultant compact. On the other hand, a bonded magnet is produced by compression-molding or injection-molding a mixture (i.e., a compound) of a powder of an R—Fe—B based magnet alloy (with particle sizes of about 100 μm) and a binder resin.
The sintered magnet is made of a powder with relatively small particle sizes, and therefore, the respective powder particles thereof exhibit magnetic anisotropy. For that reason, an aligning magnetic field is applied to the powder being compacted by the press machine, thereby making a powder compact in which the powder particles are aligned with the direction of the magnetic field.
The powder compact obtained in this manner is then sintered normally at a temperature of 1,000° C. to 1,200° C. and then thermally treated if necessary to be a permanent magnet. In the sintering process, the atmosphere is often a vacuum atmosphere or an inert atmosphere to reduce the oxidation of the rare-earth element.
To make the bonded magnet exhibit magnetic anisotropy on the other hand, the hard magnetic phases in the powder particles used should have their easy magnetization axes aligned in one direction. Also, to achieve coercivity to a practically required level, the crystal grain size of the hard magnetic phases that form the powder particles should be reduced to around the single domain critical size. For these reasons, to produce a good anisotropic bonded magnet, a rare-earth alloy powder that satisfies all of these conditions needs to be obtained.
To make a rare-earth alloy powder for an anisotropic bonded magnet, an HDDR (hydrogenation-disproportionation-desorption-recombination) process is generally adopted. The “HDDR” means a process in which hydrogenation, disproportionation, desorption and recombination are carried out in this order. In the known HDDR process, an ingot or powder of an R—Fe—B based alloy is maintained at a temperature of 500° C. to 1,000° C. within an H2 gas atmosphere or a mixture of an H2 gas and an inert gas so as to occlude hydrogen into the ingot or the powder. After that, the desorption process is carried out at the temperature of 500° C. to 1,000° C. until either a vacuum atmosphere with an H2 pressure of 13 Pa or less or an inert atmosphere with an H2 partial pressure of 13 Pa is created and then a cooling process is carried out.
In this process, the reactions typically advance in the following manner. Specifically, as a result of a heat treatment process for producing the hydrogen occlusion, the hydrogenation and recombination reactions (which are collectively referred to as “HD reactions” that may be represented by the chemical reaction formula: Nd2Fe14B+2H2→2NdH2+12Fe+Fe2B) advance to form a fine structure. Thereafter, by carrying out another heat treatment process to produce the desorption, the desorption and disproportionation reactions (which are collectively referred to as “DR reactions” that may be represented by the chemical reaction formula: 2NdH2+12Fe+Fe2B→Nd2Fe14B+2H2) are produced to make an alloy with very fine R2Fe14B crystalline phases.
An R—Fe—B based alloy powder, produced by such an HDDR process, exhibits high coercivity and has magnetic anisotropy. The alloy powder has such properties because the metallurgical structure thereof substantially becomes an aggregate structure of crystals with very small sizes of 0.1 μm to 1 μm. Also, if the reaction conditions and composition are selected appropriately, the easy magnetization axes of the crystals will be aligned in one direction, too. More specifically, the high coercivity is achieved because the grain sizes of the very small crystals, obtained by the HDDR process, are close to the single domain critical size of a tetragonal R2Fe14B based compound. The aggregate structure of those very small crystals of the tetragonal R2Fe14B based compound will be referred to herein as a “recrystallized texture”. Methods of making an R—Fe—B based alloy powder having the recrystallized texture by the HDDR process are disclosed in Patent Documents Nos. 1 and 2, for example.
A magnetic powder made by the HDDR process (which will be referred to herein as an “HDDR powder”) is normally mixed with a binder resin (which is also simply referred to as a “binder”) to make a compound, which is then either compression-molded or injection-molded under a magnetic field, thereby producing an anisotropic bonded magnet. The HDDR powder will usually aggregate after the HDDR process. Thus, to use the powder to make an anisotropic bonded magnet, the aggregate structure is broken down into the powder again. For example, according to Patent Document No. 1, the magnet powder obtained preferably has a particle size of 2 μm to 50 μm. In Example #1 of that document, an aggregate structure obtained by subjecting a powder with a mean particle size of 3.8 μm to the HDDR process is crushed in a mortar to obtain a powder with a mean particle size of 5.8 μm. Thereafter, the powder is mixed with a bismaleimide triazine resin and then the compound is compression-molded to make a bonded magnet.
On the other hand, a technique for aligning an HDDR powder and then turning the powder into a bulk by a hot compaction process such as a hot pressing process or a hot isostatic pressing (HIP) process was proposed in Patent Document No. 3, for example. By adopting a hot compaction process, the density of the powder can be increased at low temperatures. As a result, a bulk magnet can be produced with the recrystallized texture of the HDDR powder maintained.
Various other methods for producing an R—Fe—B based permanent magnet by taking advantage of features of the HDDR process have also been proposed. For example, according to the method disclosed in Patent Document No. 4, an R—Fe—B based alloy that has been prepared by melting materials in an induction melting furnace is subjected to a solution treatment, if necessary, cooled, and then pulverized into a coarse powder. The powder is further pulverized finely to a size of 1 μm to 10 μm using a jet mill, for example, and then compacted under a magnetic field. Thereafter, the green compact is sintered at a temperature of 1,000° C. to 1,140° C. within either a high vacuum or an inert atmosphere. Then, the sintered compact is kept heated to a temperature of 600° C. to 1,100° C. within a hydrogen atmosphere and then thermally treated within a high vacuum, thereby reducing the size of the main phase to 0.01 μm to 1 μm.
On the other hand, according to the method disclosed in Patent Document No. 5, first, a fine powder with a particle size of less than 10 μm, obtained by pulverizing an alloy that has been subjected to a homogenization process with a pulverizer such as a jet mill, is compacted under a magnetic field to obtain a powder compact. Then, the powder compact is treated at a temperature of 600° C. to 1,000° C. within hydrogen and then at a temperature of 1,000° C. to 1,150° C. This series of processes carried out on the powder compact corresponds to the HDDR process. In this case, however, the temperature of the DR process is higher than that of the HD process. According to the method disclosed in Patent Document No. 5, sintering process is advanced by the DR process at the higher temperature, and therefore, the powder compact can be sintered as densely as it has been. Patent Document No. 5 says that the sintering process should be carried out at a temperature of at least 1,000° C. to make a sintered body with high density.
Furthermore, according to the method disclosed in Patent Document No. 6, first, the alloy is coarsely pulverized to a mean particle size of 50 μm to 500 μm by a hydrogen occlusion decrepitation process. Thereafter, the coarse powder is compacted into a predetermined shape (under a magnetic field, if necessary) to obtain a powder compact. Then, the powder compact is subjected to the known HDDR process. And the resultant powder compact is dipped or immersed in a resin, thereby producing a bonded magnet.
According to the methods disclosed in Patent Documents Nos. 5 and 6, the powder compact is subjected to the HDDR process in both cases. However, according to the method of Patent Document No. 5, the mechanical strength is increased by increasing the density through a high-temperature sintering process. On the other hand, according to the method disclosed in Patent Document No. 6, the mechanical strength is increased by using a resin.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 1-132106    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2-4901    Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 4-253304    Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 4-165012    Patent Document No. 5: Japanese Patent Application Laid-Open Publication No. 6-112027    Patent Document No. 6: Japanese Patent Application Laid-Open Publication No. 9-148163